In various analytical methods, i.e. chromatography, online measurement and others, many scientific instruments need fluid controlling device. Most of the time, this is achieved by using different types of fluid flow path switching valves. As non-limitative examples, the function of these valves could be for sample injection, sample stream selection, fluid redirection, fraction collection, solvent selection, separation column selection or combination and other fluid switching flow paths required to realize a particular analytical method.
In these systems, the fluid pressure range could be from vacuum to value in the neighbourhood of 10,000 psig. The fluid phase could be gas or liquid. For the accuracy, precision and repeatability of the analytical method in the determination of impurities to be identified and quantified, it is of prime importance that the valves used in such method fulfil the most stringent parameters like inboard contamination, cross-port leak, leak from the inside to the outside of the system, dead volume, inertness and adsorption. In some cases the valve should be able to operate at high temperature, like 400° C., without the loss of its characteristic.
Several of these analytical methods are used in scientific instruments dedicated to be embedded in industrial process control equipment. In such applications, the analytical system must operate continuously and without human intervention. To realize an analytical system and method that meets these criteria, valves must be able to work appropriately for a long period of time, preferably two years or more, before any maintenance needs to be done on them.
Hereinbelow, several analytical method examples will be explained to help the reader understand how valve characteristic could affect overall system performance. They are not exclusive and there are dozens of valve and column combinations based on analytical methods used for any particular application. However, in all possible methods, the valve characteristic is a key parameter for system performance.
Referring to FIGS. 1A to 1D, there is shown a prior art six-ports valve used in a gas chromatography method. This is the simplest chromatography application. The sample to be analyzed flows into the sample loop. The separation column and the detector are swept by a very pure carrier gas, as illustrated in FIGS. 1A and 1B. When the valve's rotor is rotated on stator surface, the new groove alignment results in a new fluid flow path, as shown in FIGS. 1C and 1D. This position is commonly named the “sample injection” position. In this position, the sample loop content is carried to the separation column and then to the detector by the carrier gas. The various impurities are separated on the separation column and independently generate a signal from the detector having the shape of a Gaussian peak. The surface of this peak is integrated to calculate its area, by the supporting hardware and software, commonly know as an “Integrator”. The computed area is then scaled to report the quantity of impurities in some engineering unit. The valve is then restored to the sampling position shown in FIGS. 1A and 1B to start a new analyzing cycle.
FIGS. 2A to 2C show another common configuration using two six-ports valves and two separation columns. This configuration is often used when the sample matrix, i.e. sample background, is different from the carrier gas. In this case, if the sample background reaches the detector, a huge peak will result, masking some of the impurities of interest, and some types of detectors could be damaged by overloading. To avoid this, most of the sample background is first “heartcuted” or vented outside the system by the first column. This is achieved by rotating the rotor of valve V1 in the sample injection position to inject a sample, as illustrated in FIG. 2B. Then, at the appropriate time, valve V2's rotor is rotated to direct the effluent coming out of the first column outside the system, as illustrated in FIG. 2C. The valve V2 is then restored to its original position when most parts of sample background have been vented and before impurities of interest come of the first column. Then, in the second column, which is an analytical column, the impurities will be separated and sequentially introduced in the detector. No detector overload will occur since little or no sample background is present.
There are many two or multi-positions rotary valves on the market, all of them having a stator and a rotor, these two parts generally consisting of a planar surface. Most of the time, one planar surface is harder than the other one. For the sake of the discussion, see FIGS. 1A to 1D, which show a typical sample injection rotary valve used in chromatography. The fluid flow path is changed by turning the rotor on stator surface. FIGS. 1A and 1B show the valve in sampling position while FIGS. 1C and 1D show the valve in sample injection position. The sealing action is provided by strongly pressing the rotor on the stator surface. Most of the time, the rotor is made of a softer material than the one of the stator. The stator is generally polished in order to get a flat surface and minimum roughness. Different types of materials have been used for stator and rotor, i.e. metal, ceramic and various polymers. When fluid is liquid, leaks are much lower than when the fluid is gaseous, even for the same operating pressure. Molecule sizes are much bigger and their shapes much more complex for liquids than gases.
In chromatograph applications using liquid media, operating pressures are quite high, sometimes up to 10,000 psig. Such high operating pressure requires a good sealing surface to minimize leaks.
For gaseous applications, the operating pressure is much lower and most of the time below 300 psig and typically 100 to 150 psig. However, when the carrier or sample is H2 or He, a good sealing is extremely difficult to achieve.
The diameter of a He molecule is about 0.26 nm. The smallest scratch on the stator or rotor surface resulting from surface finish imperfection will cause leaks from port to port. The surface finish can be seen as a network of grooves with a random distribution. This makes it difficult to get good sealing for long periods of time. Nowadays, analytical methods and systems in which such valve is used are more efficient. This means that the total analytical cycle time has been cut in some cases by a factor of ten. The valves are therefore actuated much more often, their lifetime is then reduced and frequent maintenance is required. As reported in U.S. Pat. No. 6,453,946, such maintenance was previously required every six months, but it may now be required every week. Equipment downtime is undesirable.
In laboratory environment, frequent downtime could be at the limit acceptable. In this environment, there are always technicians to take care of analytical equipment and to reconfigure them for a new analytical method. However, for process chromatograph, frequent downtime is a serious problem. Process gas chromatograph must operate continuously as stand-alone unit. The analytical results of process gas chromatograph are the inputs of complex process control loop. When a valve slowly begins to leak, the analytical results become unstable and inaccurate. This may have a dramatic effect on a particular manufacturing process.
In rotary valves used in prior art, there is a fixed and a movable part, commonly known as stator and rotor. An example of such assembly is shown in FIGS. 3A to 3D. Generally, the rotor has some channels therein to allow for various gas connections of stator ports. The change in fluid flow path is done by turning the rotor on the stator surface. The rotation movement changes the rotor channels position seen by stator's ports. Thus, different flow paths can be achieved by changing channels configuration in the rotor and the number of ports in the stator.
Referring now to FIGS. 4A to 4D, there is shown two configurations for 10 and 12-ports valves respectively. FIGS. 5A and 5B show a configuration for sample stream selection. These configurations are not limitative or exclusive and many others could be done.
There are several embodiments of rotary valve systems known in the art. Some of them are designed simply for sample loop injection, others for syringe sample loading and others for multi-positions flow path switching. The port numbers vary from 4 to typically 12. For sample stream selection, the number of ports could be higher. All of them suffer from fast wearing caused by particle contamination, or simply by the friction between the various planar surfaces. There are no means to prevent or delay cross-port flow contamination over the time. Such rotary valve systems are disclosed in the following U.S. Pat. Nos. 3,203,249; 3,223,123; 3,297,053; 4,068,528; 4,182,184; 4,242,909; 4,243,071; 4,393,726; 4,476,731; 4,506,558; 4,577,515; 5,207,109; 5,803,117; 6,012,488; 6,155,123 and 6,672,336. All of them rely on flat surface sealing that lasts, at the best, around 9 months.
In the art, there are some valves that have a conical shape, as shown in FIGS. 6A and 6B, or a spherical shape, but they all suffer from the same problems. The conical valve concept shown in FIGS. 6A and 6B is largely used in most laboratory chromatographs. This valve is manufactured by the Valco Company and U.S. Pat. No. 4,222,412 illustrates such a valve.
An early attempt to fix one of the pre-cited problem, i.e. in this case, inboard or outboard leak, is shown in U.S. Pat. No. 2,519,574. Even if the described rotary 4-way valve is not specifically designed to be used in analytical systems, the concept shown could nevertheless be applied to it. The circular fluid O-ring type seal shown between the two planar surfaces will avoid leaking from the interior of the valve to the exterior of it and prevent inboard contamination too. However, this type of seal requires frequent replacement. Sealing effect relies on constant pressure applied by both planar surfaces on the seal, particle contamination causes seal wearing and leaks occur. The material used (generally elastomer but others are possible) could also desorb or adsorb some sample molecules when the pressure and/or temperature are changed. Furthermore, no means are provided to avoid cross-port leaks when the surface becomes scratched by the fluid's particle or by particles coming from the seal wearing.
Also known in the art, there is U.S. Pat. No. 5,193,581, which describes a way to eliminate the contamination of a selected sample by the unselected sample streams. There is an evacuation groove in the rotor that will carry away the leak coming from unselected channels, however there are serious drawbacks. This method does not fix the problem of cross-port leak between unselected ports. This is very important if various samples are reactive and non compatible. There is also a dead volume in the rotor. There is also an O-ring between the rotor and valve housing acting as a seal, so out-gassing could occur and O-ring wearing will cause leak.
Also known in the art, there is U.S. Pat. No. 6,067,864, which also describes a rotary sample selection valve that tries to eliminate the contamination of the selected sample by the unselected ones. The method uses a vacuum source to evacuate all the unselected channel through a common port. There is always a positive pressure differential between selected channels and the unselected evacuation volume. However, there is also a serious drawback since the system uses O-ring for sealing. So, out-gassing will occur as well as leaks because of wearing. Furthermore, all unselected sample streams must be compatible, since they are mixed together.
Also known in the art, there is U.S. Pat. No. 6,453,946, which describes a method to extend the valve's life. This method suggests the use of vespel as material for the rotor and stainless steel coated with tungsten carbide/carbon (WC/C). Even if this method helps to have a longer lifetime before leaks occur, it will not last two or three years. They report 200,000 cycles, however, valve actuated every two minutes in a process gas chromatography system will have more than 200,000 cycles after a year. Leaking will therefore occur and maintenance will be required.
Thus, a rotary valve overcoming the drawbacks of the existing ones while providing the long lifetime needed in process analytical equipment would be desirable.